A Magnetic Resonance (MR) imaging system generally includes a main magnet, gradient coils (x, y and z), an RF system, a transmit coil, and a receive coil. The main magnet polarizes protons in an object or subject (e.g., a human or animal patient) in an examination region. The gradient coils localize and spatially encode the positions of the protons. The RF system generates an excitation signal, and the transmit coil excite protons based on the excitation signal. The receive coil receives a MR signal produced in response to the protons returning to the pre-excite state. The transmit and receive coils may be the same coil or different coils. The received MR signal is processed and the processed data is used to generate one or more images of the scanned region of the object or subject.
In one MR system, the RF system includes a signal modulation and generation section electrically connected to a signal amplification section. The signal modulation and generation section creates a modulated digital waveform and generates an analog RF signal therefrom based on a predetermined carrier description and envelope. The analog RF signal is transmitted to the signal amplification section. The signal amplification section amplifies the analog RF signal, and the amplified analog RF signal is provided to the transmit coil. With this system, the signal amplification section is assumed to be “ideal”, in that the signal amplification section is expected to amplify the analog input RF signal with high fidelity and provide an accurate high power replica of the low power input to the coil.
Unfortunately, the analog RF signal fed to the signal amplification section may not be “ideal” for the transmit coil in that its output to the coil may not follow the behavior of the low level input and the predetermined carrier description and envelope with sufficient accuracy for the application. As such, the signal amplification section may require additional analog circuitry which dynamically modifies the amplifier behavior or analog signals in order to deliver the desired high power analog output. The analog RF signal output may deviate from the desired output due to variations in the amplifier phase and gain due to various operating and environmental conditions (e.g., temperature, drift, output power level, etc.).
Current compensation techniques include detection of the actual output signal and/or various amplifier operating and environmental conditions, and using these measurements to implement corrections which tend to reduce the output errors. Compensation techniques may include modification of the amplifier characteristics (e.g., through changes in amplifier gain and phase), closed loop feedback, which modifies the applied input signal to the main amplifier based on a comparison of the ideal input and actual output, or a combination thereof.